The use of alloantisera to 'Ly' surface components which are expressed exclusively on murine T cells represents a new and direct approach to the separation of T cell subclasses and analysis of their role in immune responses. Our current understanding of the immune functions of T cell subclasses is based mainly on a strategy which correlates in vitro T lymphocyte function with cell surface phenotype. This approach had yielded a clearer picture of the division of labor among lymphocytes engaged in cytotoxic and proliferative responses to alloantigens and 'altered self', helper and suppressive function and 'natural killer' activity. This method of identifying and separating functional T cell subclasses has also permitted more direct approach to the characterization of the cell-free factors responsible for some of these biologic effects. The purpose of this project is to establish the contributions of these different T cell subclasses, which have so far been defined in vitro, to in vivo cellular mechanisms that govern classical transplantation reactions such as GVH disease and homograft rejection, reactivity to infectious agents as well as the immune response to tumors.